In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in the manufacturing processes. Specifically, non-destructive evaluation (NDE) methods must assess the structural integrity of composite materials. Conventional NDE methods are slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures.
Various methods and apparatuses have been proposed to assess the structural integrity of composite structures. One method generates and detects ultrasound using lasers. A pulsed laser beam generates ultrasound on a work piece, while a second laser beam illuminates the work piece. Surface displacements generated by the generation laser modulate the illumination laser beam, and the modulated laser energy is collected with collection optics. The modulated light is processed to extract useful information about the structural integrity of the target.
One advantage provided by such a laser ultrasound inspection is the ability to perform ultrasonic measurements without mechanically coupling or contacting the target to be inspected. Additionally, laser ultrasound may provide low sensitivity to the orientation of the sample relative to the illuminating laser beam. These abilities make laser ultrasound highly useful in the inspection of parts.
Laser ultrasound requires a line of sight for the laser to carry out the measurement. However due to the complex shapes often inspected, it is difficult to realize a line of sight from the laser source to the sample being inspected. One solution brings optical fibers near the area to be inspected. In such a case, multiple optical fibers transport multiple lasers to generate laser ultrasound and illuminate the ultrasonic displacements at the target. The fibers also serve to collect phase modulated light scattered at the target. In this arrangement, different optical setups may be used for each optical fiber. These different optical setups can cause the laser ultrasound probe to become cumbersome. Additionally, system optics associated with ultrasound generation, detection, and collection in the path of one another may decrease the optical efficiency of the system while increasing the size and complexity.
One solution to uses separate devices for ultrasound generation, illumination and collection. However, the use of multiple devices increases the time to perform inspections, requires multiple operators working together to take measurements, requires more powerful and expensive lasers, and results in lower measurement accuracy.
When direct line of sight is not available optical fibers can be used to bring the laser light at a condition from which the inspection can be carried out. Usually this requires the use of multiple optical fibers. The use of the multiple optical fibers often requires different optical setups for each optical fiber. Multiple optical fiber probes historically have made the remote access laser ultrasound head bulky and not optically efficient as their optics interferes with one another. A decrease in optical efficiency very often compensated for by increasing the size of the collection optics for the power associated with the lasers. However this solution makes the laser ultrasound probe larger than during access to confined spaces more difficult.
Fiber optics laser ultrasound heads have been designed for the purpose of remote access laser ultrasound inspection using individual optics for each optical fiber. These probes are bulky and require high-powered detection in order to compensate for the low collection efficiency. These probes can be made less bulky by using a single fiber for the laser illumination and light collection. However, this configuration tends to produce high parasitic noise due to back reflections. Another alternative solution utilizes piezoelectric transducers to generate ultrasound. This solution is not always attractive as this requires mechanical contact with the inspection area and a very high degree of control associated with the orientation of the transducer relative to the inspection area (i.e. the transducer must be normal to the inspected surface). These requirements make inspection difficult, slow, and expensive. Additionally, the use of transducers requires the use of wires physically coupled to the transducer for power, etc. In some cases, measurements must be performed in flammable environments where no wire may be brought near the inspection area. For these reasons, optical fibers are more desirable. Therefore, a need exists for a more effective system and method to perform laser ultrasonic testing in confined spaces or on complex work pieces.
Therefore, a need exists for a more effective system and method to perform laser ultrasonic testing in confined spaces or on complex work pieces.